This invention relates to synthetic ester lubricant base stocks, and more particularly to high molecular weight synthetic ester lubricant base stocks formed from neopentylpolyols and monocarboxylic acids or mixtures of monocarboxylic acids suitable for use in industrial refrigeration systems.
It is well known that chlorofluorocarbons previously utilized as propellants in aerosols and as refrigeration fluids are no longer used due to the adverse impact on the earth""s ozone layer. Efforts to eliminate the use of chlorofluorocarbons has led to the development of alternative fluids that are highly or fully fluorinated hydrocarbons.
Working refrigeration fluids generally include a minor amount of lubricant. Typically, the refrigeration fluids include between about 5 to 15 or 20 parts by weight of lubricant with the balance being the fluorocarbon. In chlorofluorocarbon systems, lubricants have been mineral oils which are fully miscible with the chlorofluorocarbons. However, such lubricants are not miscible with the highly or fully fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane, commonly known as HFC-134a. Other chlorine-free fluorocarbons include a hydrofluorocarbon (HFC) having 1 to 3 carbon atoms and preferably 1 to 2 carbon atoms, for example, difluoromethane (HFC-32), trifluoromethane (HFC-23), pentafluoroethane (HFC-125), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1-trifluoroethane (HFC-143a), 1,1-difluoroethane (HFC-152a) or a mixture of two or more kinds of these HFCs.
It has been found that certain carboxylic esters of polyhydroxy compounds are fully miscible with the highly or fully fluorinated fluids. An early publication describing such lubricants is European Patent EP 0 422 185 B1. Lubricants of the type disclosed are low viscosity fluids of the type utilized in home window air conditioning units and automotive air conditioning units. The lubricants are prepared by the condensation of pentaerythritol and monocarboxylic acids containing between about 6 to 8 carbon atoms. This generally produces an ester having a viscosity at 40xc2x0 C. of between about 17.5 to about 52.0 cSt. Such lubricants are also disclosed in U.S. Pat. No. 5,964,581.
Larger industrial refrigeration units of the type utilized in building and factory cooling units call for refrigeration fluids including lubricants having significantly higher viscosity in the ISO 100 to 350 range (100 to 350 centistokes at 40xc2x0 C.). Typically such higher viscosity lubricants would be formed from dipentaerythritol and higher molecular weight carboxylic acids. This would include lubricant base stocks having a viscosity between about 68 to 400 cSt at 40xc2x0 C. Alternatively, dicarboxylic acids can be utilized along with the monocarboxylic acids to form complex esters when reacted with a polyol, however these esters tend to be less stable than polyol esters of monocarboxylic acid alone. Production of higher viscosity lubricants is generally not possible from a monopentaerythritol starting material. It is preferable to use monopentaerythritol, however, because the cost of monopentaerythritol is much lower than the cost of dipentaerythritol.
Accordingly, it is desirable to provide a synthetic ester lubricant base stock having a viscosity between about ISO 68 to 400 based on condensation of monopentaerythritol and monocarboxylic acids.
Generally speaking, in accordance with the invention, high viscosity synthetic ester lubricant base stocks based on polyneopentylpolyol esters suitable for use with high-fluorine containing refrigeration fluids are provided. The polyneopentylpolyol esters are formed by reacting a neopentylpolyol having less than 10 carbon atoms with at least one monocarboxylic acid selected from the group of linear acids having from 4 to 10 carbon atoms, and branched chain acids having from 5 to 10 carbon atoms. The neopentylpolygls include pentaerythritol, trimethylolpropane, trimethylolethane, neopentyl glycol and the like. A preferred acid is a mixture of a linear acid and a branched acid in a mole ratio of between 5:1 to 1:5, preferably between about 1:1 to 1:3 and most preferably between 1:3 to 1:4.
The polyol and acid are first reacted in the presence of an excess of hydroxyl groups to form a partial polyneopentylpolyol ester of desired viscosity. Then an excess of monocarboxylic acid or acid mixture is added to the reaction mixture and the esterification reaction is continued to yield an ester having a viscosity in the range of 68 to 400 cSt at 40xc2x0 C.
Optionally, finished esters can be prepared to a higher viscosity than desired. These can then be blended with a lower viscosity neopentylpolyol ester to achieve the desired target viscosity. Significant economic benefits are realized by this approach to building viscosity in the ISO range of between about 68 to 400. The base stocks are then blended with a refrigerant lubricant additive package to form the lubricant. The additive packages are well known in the art and include antioxidants, anti-wear agents, metal pacifiers, friction modifiers and the like.
In a preferred embodiment of the invention, the starting polyol is pentaerythritol and the acid is a mixture of valeric acid (C5 acid) and 3,5,5-trimethylhexanoic acid (isoC9 acid). It has been found that these esters are particularly suitable for use in synthetic refrigeration fluids as they provide good miscibility with highly or fully fluorinated refrigeration fluids over a broad temperature range.
Accordingly, it is an object of the invention to provide a high viscosity synthetic ester base stock having satisfactory miscibility with refrigeration fluids.
Another object of the invention is to provide an improved high viscosity synthetic ester base stock formed from a neopentylpolyol at reduced costs.
A further object of the invention is to provide a high molecular weight synthetic ester base stock having a high ISO grade in the range of about 68 to 400 based on a neopentylpolyol.
Yet another object of the invention is to provide a method for preparing a high viscosity synthetic ester base stock from a neopentylpolyol and an acid or acid mixture consisting of monocarboxylic acids.
Yet a further object of the invention is to provide a high viscosity synthetic ester base stock formed from monopentaerythritol and an acid mixture of valeric acid and isoC9 acid.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. The invention accordingly comprises a composition of matter possessing the characteristics, properties and the relation of components which will be exemplified in the compositions hereinafter described, and the scope of the invention will be indicated in the claims.
The high viscosity synthetic ester lubricant base stocks prepared in accordance with the invention are esters or blends of esters of certain polyneopentylpolyols. These esters provide good miscibility with highly or fully fluorinated refrigeration working fluids and are obtained at significant economic benefit.
In the preferred embodiments of the invention, the base stock is a blend of esters which form the reaction product of a two step process. In the initial step a neopentylpolyol having less than 10 carbon atoms and preferably from 5 to 8 carbon atoms is reacted with an acid or acid mixture of linear C4-C10 acid and/or branched C5-C10 acid in the presence of an excess of hydroxyl groups to carboxyl groups in the presence of an acid catalyst to form a partial polyneopentylpolyol ester mixture. When the anticipated water of reaction has been removed from the reaction vessel to yield a product of desired viscosity, the resulting partial polyneopentylpolyol ester is then reacted with an excess of an acid or acid mixture that can vary or be the same acid or acid mixture used in the initial step to yield the high viscosity ester product.
The neopentylpolyol utilized to prepare compositions in accordance with the invention preferably is at least one neopentylpolyol represented by the structural formula: 
wherein each R is independently selected from the group consisting of CH3, C2H5 and CH2OH. Examples of such a neopentylpolyol include pentaerythritol, trimethylolpropane, trimethylolethane, neopentyl glycol and the like. In some embodiments of this invention, the neopentylpolyol comprises only one such neopentylpolyol. In other embodiments it comprises two or more such neopentylpolyols. When the neopentylpolyol is pentaerythritol, the polypentaerythritol moiety of the reaction product includes pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol, etc.
The acid or acid mixture includes a linear monocarboxylic acid and/or a branched chain monocarboxylic acid. The linear acid is one having from 4 to 10 carbon atoms and the branched acid has from 5 to 10 carbon atoms. Preferably, the acid is valeric acid (C5 acid) or a mixture of acids of valeric acid and 3, 5, 5-trimethylhexanoic acid (isoC9 acid).
When an acid mixture is used, preferably the linear acid is valeric acid present in amounts between about 15 to 35 mole percent and the branched acid is isoC9 acid present between about 65 to 85 mole percent. In a most preferred embodiment of the invention the valeric acid is present between about 20 to 25 mole percent and the iso-C9 acid between about 75 to 80 mole percent.
The initial stage of the reaction to form the polyneopentylpolyol esters is conducted in the manner described by Leibfried in U.S. Pat. No. 3,670,013 and in commonly assigned U.S. Pat. No. 5,895,778. The descriptions of both patents are incorporated herein by reference. Here, when pentaerythritol is the neopentylpolyol, a reaction mixture of pentaerythritol (272 w) and valeric acid (217 v) is placed into a reactor with extra valeric acid (38 v) in a receiver to assure a constant level of valeric acid in the reaction mixture. The mixture is heated to a temperature of 171xc2x0 C. and concentrated sulfuric acid (1.0 w) diluted with water (2 v) is added. The reaction mixture is heated to 192xc2x0 C. and maintained until 50.5 v of water is removed after about 1.4 hours. The Leibfried analysis of the product shows pentaerythritol, dipentaerythritol, tripentaerythritol and tetrapentaerythritol moieties at weight ratios of 34:38:19:8.
The initial concentration of monocarboxylic acid material in the reaction mixture is such as to provide an initial mole ratio of carboxyl groups to hydroxyl groups in a range from about 1:4 to about 1:2.
In the present case, when the polyol is pentaerythritol, the polyol and carboxylic acid or acid mixture are charged in a mole ratio of about 1:1 to 1:3 with a preferred starting ratio of about 1:2. This mixture is reacted using an effective amount of an acid catalyst material as described in the Leibfried patent.
The reaction is continued until the desired quantity of water is removed from the reaction mixture. This is determined by experimentation and may be estimated by calculating the expected amounts of water of reaction. At this point when the starting neopentylpolyol is pentaerythritol, the mixture includes partial esters of pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol and the like. Optionally, the acid catalyst may be neutralized with alkali. In order to complete the esterification of the partial esters, an excess of an acid or acid mixture and optionally an esterification catalyst as is well known in the art is added to the reaction mixture which is then heated, water of reaction removed and acid returned to the reactor. Optionally, the product can be isolated without completing the esterification of the remaining hydroxyl groups, or the esterification can be carried out to some desired extent other than to completion.
The acid catalyst is at least one acid esterification catalyst. Examples of acid esterification catalysts include mineral acids, preferably, sulfuric acid, hydrochloric acid, and the like, acid salts such as, for example, sodium bisulfate, sodium bisulfite, and the like, sulfonic acids such as, for example, benzenesulfonic acid, toluenesulfonic acid, polystyrene sulfonic acid, methylsulfonic acid, ethylsulfonic acid, and the like. The reaction mixture is heated to between about 150xc2x0 and 250xc2x0 C. while withdrawing acid vapor and water vapor to yield the polypentaerythritol partial ester product.
Prior to esterifying the partial esters, the intermediate product will include a variety of condensation products of the neopentylpolyol. When pentaerythritol is the neopentylpolyol, the reaction mixture will include significantly more pentaerythritol than the 10 to 15 weight percent generally present in commercially available dipentaerythritol. Depending on the initial ratio of carboxyl groups to hydroxyl groups and selection of reaction conditions, the partial ester product may include the following components in the weight ranges specified in the following table.
In the initial step of the reaction an excess of hydroxyl groups to monocarboxylic acid groups is present to form the partial esters of the neopentylpolyol, such as partial esters of pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol, etc. The excess of hydroxyl groups is necessary to promote the polymerization of the partial esters. The molar ratio of acid or acid mixture to the polyol can be varied depending on the desired rate of reaction, degree of condensation and the ultimate desired viscosity of the lubricant. After formation of the partial esters, generally, a 10 to 25 percent excess, with respect to hydroxyl groups, of an acid or acid mixture is added to the reactor and heated. Water of reaction is collected during the reaction while the acids are returned to the reactor. The use of vacuum will facilitate the reaction. When the hydroxyl value is reduced to a sufficiently low level, the bulk of the excess acid is removed by vacuum distillation. Any residual acidity is neutralized with an alkali. The resulting polyneopentylpolyol ester is dried and filtered as described in Example 1 below.
The invention will be better understood with references to the following examples. These examples are presented for purposes of illustration only, and are not intended to be construed in a limiting sense.
The reactor in each preparatory example is equipped with a mechanical stirrer, thermocouple, thermoregulator, Dean Stark trap, condenser, nitrogen sparger, and vacuum source. The esterification may or may not be carried out in the presence of esterification catalysts, which are well known in the art.